deletions | additions       diff --git a/introduction_1.tex b/introduction_1.tex  index 36115e1..22a2907 100644  --- a/introduction_1.tex  +++ b/introduction_1.tex 
...
The traditional ISM-based probes of the Milky Way's structure have been HI and CO. Emission in these tracers gives line intensity as a function of velocity, so the position-position-velocity data resulting from HI and CO observations can give three dimensional views of the Galaxy, if a rotation curve is used to translate line-of-sight velocity into a distance. Unfortunately, though, the Galaxy is filled with HI and CO, so it is very hard to disentangle features when they overlap in velocity along the line of sight. Nonetheless, much of the basic understanding of the Milky Way's spiral structure we have now comes from HI and CO observations of the Galaxy, much of it from the compilation of CO data presented by \citet{Dame2001}.  Recently, several groups have targeted high-mass star-forming regions in the plane of the Milky Way for high-resolution ISM observations. observation.  In their BeSSeL Survey, Reid et al. are using hundreds of hours of VLBA time to observe hundreds of regions for maser emission, which can give both distance and kinematic information for very high-density ($n>10^8$ cm$^{-3}$) gas \citep{Reid2009,Brunthaler2011}. In the HOPS Survey, hundreds of positions associated with the dense peaks of infrared dark clouds have now been surveyed for ${\rm NH}_3$ emission \citep{Purcell2012b}, yielding high-spectral resolution velocity measurements towards gas whose density typically exceeds $10^4$ cm$^{-3}$. In follow-up spectral-line surveys to the ATLASGAL \citep{Beuther2012a} dust-based survey of the Galactic Plane, \citet{Wienen2012} have measured ${\rm NH}_3$ emission in nearly 1000 locations. The ThrUMMs Survey aims to map the entire fourth quadrant of the Milky Way in CO and higher-density tracers \citep{BarnesPeter2010}, and it should yield additional high-resolution velocity measurements. Targets in high-resolution (e.g. BeSSeL) studies are usually identified based on continuum surveys, which show the locations of the highest column-density regions, either as extinction features (``dark clouds" in the optical, ``IRDCs" in the infrared), as dust emission features (in surveys of the thermal infrared), or as gas emission features (e.g. HII regions).